How come Carbon-13, one of the stable isotopes of Carbon, has an atomic mass of 13.00335 amu and not 13 amu? The nucleus of a Carbon-13 atom consists of 6 protons and 7 neutrons, so why shouldn't the atomic mass be 13? And, furthermore, if it isn't 13 amu, then why is it called Carbon-13? Why not Carbon-13.00335?

Basically, the amu of Carbon-13 is known as 13. Again, in some places, the atomic mass of Carbon-13 is given 13.00335 amu. And, whenever we try to get the weighted average of all the isotopes of Carbon, we use 13.00335 as the atomic mass of Carbon-13.

It is too confusing. Simply, horribly confusing!

Someone please help me! I know that it might be a difficult question to ask, but if some confusions are never solved, it will stay mysterious forever.

It's not just Carbon-13. There are many other isotopes.

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Answers

2016-01-11T21:47:08+05:30

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It has to do something with the natural abundance of the isotope. Still 13.0035 has a very negligible amount 0.0035 so we can take 13
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2016-01-12T11:46:37+05:30

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In basic chemistry, each proton and neutron is said to have a mass of 1 amu.  However, the original mass of both the subatomic particles is slightly higher than that.
Proton - 1.0072766 a.m.u. and Neutron - 1.0086654 a.m.u.

So, Carbon 13 in general has a mass of 13 amu when taking 1 amu as their masses. On the other hand if we use the precise masses of protons and neutrons, then the total mass of Carbon 13 comes to stand at 13.00335.

As the previous respondent has said, 0.00335 is sometimes seen as too  
negligible to be counted.

You  should note that,different books have different versions of this. It depends on you which measurement you want to use. Personally, I'd say that taking C 13's mass as 13 amu is more accepted and convenient.
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